In spite of several decades of extensive effort, protein-calorie malnutrition remains as the most important nutritional problem of the developing world. Several of the most important problems--the interrelationship between the two, the metabolic consequences of varying degrees of chronic energy and protein deficiency, and the long-term consequences of various forms of malnutrition upon behavior and learning--cannot be answered by direct studies with children. There are many reasons why these problems are best investigated with experimental animals which have nutritional needs, developmental patterns, and behavioral characteristics and learning abilities which approximate those of infants and children such as infant and young monkeys. These studies proposed are: 1) to define amino acid and protein requirements of these animals, 2) to evaluate the effects of varying degrees of energy restriction upon amino acid and protein requirements, 3) to characterize the metabolic patterns, primarily the changes in insulin, growth hormone, glucagon, cortisol, amino acid and fatty acid plasma levels, associated with varying degrees of protein, amino acid, energy restriction and of combined restriction aiming to individualize their regulatory roles, and 4) to further evaluate the persistent defects in behavior and learning already described in animals subjected to malnutrition during the first weeks of life.